Herodian (late second, first half third century): Greek historian, author of a History of the Roman Empire since the Death of Marcus Aurelius in which he describes the reign of Commodus (180-192), the Year of the Five Emperors (193), the age of the Severan dynasty (211-235), and the Year of the Six Emperors (238).
The translation was made by Edward C. Echols (Herodian of Antioch's History of the Roman Empire, 1961 Berkeley and Los Angeles) and was put online for the first time by Roger Pearse (Tertullian.Org). The version offered on these pages is hyperlinked and contains notes by Jona Lendering.
End of Plautianus
[3.12.1]  Agreeing to those proposals, the tribune made his customary rounds through the entire imperial palace without interference. But knowing that it would be impossible for him to kill two emperors, especially since they were housed in different parts of the palace, Saturninus stood outside the bedroom of Severus and, summoning the imperial household guards, demanded to be taken to the emperor so that he might give him information involving his safety. The guards reported the matter to Severus and were ordered to bring the tribune before him.
[3.12.2] The tribune approached the emperor and said: "I have come to you, Master, as the man who sent me well knows, to be your assassin and murderer, but I hope and pray that I will be instead your benefactor and savior. Plautianus, scheming to seize the empire, ordered me to murder you and your son, and he issued this order not in words alone but in writing. This memorandum is my witness. I undertook the assignment because I was afraid that if I refused it he would entrust the task to someone else, and I have come here to disclose these matters to you so that his intrigues may not remain undetected."
[3.12.3] But even though Saturninus made these charges with much weeping, Severus was not immediately convinced. On the contrary, since he had great affection for the prefect, he suspected that the affair was a trick of some sort to deceive him; he believed that his son, in his hatred of the prefect and his daughter, had contrived a slanderous and fatal plot against the man.
[3.12.4] The emperor therefore sent for his son and reprimanded him for having devised such a scheme against a man kindly disposed toward the emperor and his intimate friend as well. At first Caracalla swore on his honor that he knew nothing about what the tribune was saying; but when the man insisted and produced the memorandum, the young emperor encouraged him, urging him to prove the truth of his charges. Realizing his danger and fearing the emperor's affection for Plautianus, the tribune now knew that if the plot remained confused and unproved, he could expect a death that would not be accidental.
[3.12.5] "Master," he said, "do you wish stronger proof or clearer evidence of some sort? Then allow me to go to the front of the palace and reveal to one of the men loyal to me that the murder is done. Trusting me, Plautianus will come here in the belief that he is occupying the deserted palace. When he arrives, it will be your task to discover the truth. Order complete silence about the palace so that the plan may not be upset by being previously discovered."
[3.12.6] After making this proposal, the tribune ordered one of his most trusted men to carry a message to the prefect telling him to come to the palace as quickly as possible. The messenger was to say that both emperors were dead, and it was imperative that the prefect be inside the palace before the news was reported to the people. Then, with the Palatine Hill in his hands and the succession already settled, all the Romans, willingly or unwillingly, would offer allegiance not to an emperor to be chosen but to one already established.
[3.12.7] Believing this message, and with high hopes, Plautianus, though it was late at night, put on a breastplate beneath his robe for protection, mounted a chariot, and drove to the palace at top speed, accompanied by a few friends who were with him when the messenger came and who thought that he had received an emergency summons from the emperors.
[3.12.8] Plautianus entered the palace unchallenged, since the guards were unaware of what was taking place. The tribune came forward to meet the prefect and set a trap for him: saluting Plautianus as emperor and taking him by the hand in the customary fashion under the circumstances, Saturninus led him into the bedroom where he said the bodies of the emperors had been thrown.
[3.12.9] Severus had already alerted some of the younger bodyguards to seize the prefect as he entered the room. Then Plautianus, who had expected a far different reception, was caught and held fast. When he saw both emperors standing before him, he was terror-stricken, and pleaded with them, trying to defend himself and swearing that it was all a mistake, a plot, a conspiracy against him.
[3.12.10] When Severus reproached him with the many favors he had done him and the many honors he had awarded him, and Plautianus in his turn reminded the emperor of his previous loyalty and good will, Severus was beginning to believe the prefect until his robe fell open and revealed the breastplate beneath it. When he saw the armor, Caracalla, who was bold and quick to act and naturally hated the man, spoke up: "How would you explain these two facts?
[3.12.11] First, that you came unordered to your emperors at night, and second, that you came here wearing that breastplate? Who goes to a feast or a revel in full armor?" After saying this, Caracalla ordered the tribune and the other praetorians present to draw their swords and kill this proven enemy.
[3.12.12] Obeying without delay the young emperor's orders, they killed Plautianus and threw his body into the street, so that the affair might be clear to all and he would be vilified by those who despised him. Such was the fate of Plautianus, who, maddened by his greed to have everything, was betrayed in the end by a faithless subordinate.note[Caracalla would later order the execution of Saturninus.]